`light' if your background is light, or nil (automatic by default)
if you want Emacs to examine the brightness for you.
-If you change this without using customize after Emacs has started,
-you should use `frame-set-background-mode' to update existing frames."
+If you change this without using customize, you should use
+`frame-set-background-mode' to update existing frames."
:group 'faces
:set #'(lambda (var value)
(set-default var value)